The present invention relates to a retroactive device as part of a hydraulic servo-steering system that also includes a hydraulic servo-valve device, in particular for motor vehicles.
A device of this type is known from DE 19616439 C1. In the case of the known device, an additional, controllable restoring force is exerted onto the steering column, and therefore the steering wheel, of a motor vehicle, on deflection of the servo-valve device, in that spherical retroactive elements are pushed into V-shaped grooves by a controllable hydraulic force. In the central position, which, in the case of a motor vehicle servo-steering system, substantially corresponds to driving straight ahead with no steering torque, the restoring elements are not hydraulically acted on. This central position, which partly determines the driving feel of a motor vehicle, is, in principle, determined by the restoring force of the torsion bar of the servo-valve device. In the generic prior art, provision is made for a flat coil spring, which externally embraces the retroactive elements, to be provided for mechanically, resiliently biasing the retroactive elements into the grooves if the central position is not sufficiently defined by the rigidity of the torsion bar alone. This solution, in which the servo-valve device is subjected in the central position to a mechanical basic load, is, in practice, unsatisfactory. The generic prior art therefore proposes exerting a hydraulic basic load in that a biased one-way valve is arranged hydraulically parallel to the retroactive elements. This arrangement produces a pressure differential between an internal chamber and an external chamber of the retroactive device. Although this solution is provided for servo-steering systems comprising hydraulic pumps with a constant output flow, it may not be used for hydraulic pumps having a variably controlled output flow, since the retroaction would vary with the output flow.
Other retroactive devices are known from U.S. Pat. Nos. 5,046,573, 5,070,958 and 5,517,899. In the case of these devices, the retroactive moment is exerted by profile members, which extend in the axial direction of the rotary slide valve and into which the retroactive elements are pushed hydraulically in the axial direction. In the first two documents, the basic load is applied by means of a helical spring and a slide arranged between the pressure-side external chamber and the low pressure-side internal chamber. The slide is configured as a floating piston, which is movable in the axial direction and is adjacent, on the one hand, to the retroactive elements and, on the other hand, to the helical spring. The force of the helical spring is additive to the hydraulic force acting on the slide, and thus on the retroactive device. There is no hydraulic basic load. This solution, in which the servo-valve device is subjected in the central position to a mechanical basic load, is also, in practice, unsatisfactory.
The object of the present invention is therefore to improve the known retroactive device in such a way that a controllable or adjustable basic load may be exerted even with a varying output flow.